The present invention relates to an electro-optic display device, especially for a timepiece, of the type which includes a transparent conductive substrate and an electrochromic layer applied on said substrate, these two elements constituting together a first electrode; a counterelectrode maintained at a distance from the said first electrode and an acidic ion conductive medium located in the space between the two said electrodes, the electrochromic layer constituting a pattern when viewed in plan. In particular, the invention is directed to the provision of a protective layer in situ deposited from the ion conductive medium upon the electrochromic layer and resulting from the introduction of a doping agent to the ion conductive medium, this doping agent being the salt of a cation having an ionic radius no less than 0.99 Angstrom according to the classification of Pauling, said layer serving to inhibit continued deterioration of the electrochromic layer by the activity of the acidic ion conductive medium.
The term "electrochromic layer" is understood to define a layer made of a material whose absorption of electromagnetic radiation in a given wavelength region is altered by application of an electric field. Such materials can show a weak absorption of the visible radiation in the absence of an electric field so that the layer is almost transparent. However, when such materials are submitted to an electric field, they absorb, for example, the red end of the spectrum so that they take a blue color. Similar effects can be observed in other portions of the spectrum of the electromagnetic radiation, visible as well as invisible.
When an electrochromic layer is deposited on a transparent conductive substrate, one can vary the optical density of such layer by applying an electric field between the electrode which is constituted by the layer and the substrate and another electrode which may be termed a counterelectrode. The space between the two electrodes may be filled by an ion conductive medium with the electrodes forming a kind of sandwich therewith. The application of a potential difference across the terminals of the two electrodes causes the electrochromic layer to pass from a clear state, for example, to a dark state (colored).
This type of sandwiched display device is known and is employed in a form of a pattern with several patterns forming a numeral. A display device comprising say, seven segments, permits, by a suitable choice of these segments, a representation to be effective of all the arabic numerals from 0 to 9. When the segments alternatively are colored and uncolored, the numerals appear and disappear successively. One terms the succession of one coloration and one uncoloration a "cycle".
When one of the components of the ion conductive medium is an acid, the edges of these segments undergo progressive erosion. Consequently, after a certain number of cycles, say several hundreds of thousands, the outlines of the segments or of any other pattern, become blurred and the width of the segments decreases so that the reading or discernability thereof becomes difficult. The present invention is intended to remove this drawback.
One type of electro-optic data display imaging device that can be formed as an arrangement with an imaging area, a counterelectrode area and a suitable ion conductive medium layer sandwiched therebetween is disclosed in U.S. Pat. No. 3,704,057 granted Nov. 28, 1972 to Lindley Clair Beegle, where a layer of conducting medium is sandwiched between the electrochromic layer deposited on a transparent conductive layer and the counterelectrode. A further improvement has been disclosed in U.S. Pat. No. 3,819,252 granted June 25, 1974 to Robert Domenico Giglia, whereby the conductive medium was transformed into a semi-solid ion conductive gel by the addition of gelling agents giving rise to better switching speeds and increased lifetime.
Giglia suggested the use as a spacing layer of a semi-solid ion conductive gel, comprising in one embodiment a combination of sulfuric acid and a gellling material for the acid.
Also the spacing layer could be made ionically conductive by semi-solid material containing some ionically conducting material as one or more salts selected from group IA and IIA alkali or alkaline earth materials, smaller ions being preferred to larger ions since their ionic mobility is larger.
Although the improvement in electrode reversibility and reproducibility obtained by the use of these gels is significant, it is applicant's experience that a semi-solid medium containing one or more salts selected from group IA and IIA alkali or alkaline earth materials do not give a sufficient contrast,which should be at least 40%. An acidic medium, should, therefore, be used, but it was found by applicant that whenever acidic media are used in the abovementioned display, whether gelified or dissolved in solvents, the material forming the electrochromic layer "is attacked" and the life-time of the display amounting to 1 to 2 millions cycles for a good image quality is insufficient to make commercial watch displays which have to display hours, minutes and seconds. (Operating at lower contrast level would give longer life-time but without fulfilling the need for good legibility of a watch display.)